Re: [RFC PATCH] mfd: mfd-core: inherit only valid dma_masks/flags from parent

[Robin Murphy <robin.murphy@arm.com>]

On 2020-03-10 11:09 pm, Michael Walle wrote:
> Only copy the dma_masks and flags from the parent device, if the parent
> has a valid dma_mask/flags. Commit cdfee5623290 ("driver core:
> initialize a default DMA mask for platform device") initialize the DMA
> masks of a platform device. But if the parent doesn't have a dma_mask
> set, for example if it's an I2C device, the dma_mask of the child
> platform device will be set to zero again. Which leads to many "DMA mask
> not set" warnings, if the MFD cell has the of_compatible property set.
> 
> [    1.877937] sl28cpld-pwm sl28cpld-pwm: DMA mask not set
> [    1.883282] sl28cpld-pwm sl28cpld-pwm.0: DMA mask not set
> [    1.888795] sl28cpld-gpio sl28cpld-gpio: DMA mask not set
> 
> Thus a MFD child should just inherit valid dma_masks and keep the
> platform default otherwise.
> 
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: Rob Herring <robh@kernel.org>
> Cc: Robin Murphy <robin.murphy@arm.com>
> Cc: Russell King <linux@armlinux.org.uk>
> Signed-off-by: Michael Walle <michael@walle.cc>
> ---
> 
> Hi,
> 
> I don't know if that is the correct way of handling things. Maybe I'm
> also doing something wrong in my driver, I had a look at other I2C MFD
> drivers but couldn't find a clue why they shouldn't have the same
> problem.

The underlying issue is that about 99% of MFD children should not be 
going through dma_configure() at all because their parent 'real' device 
is not on a DMA-capable bus, but as they are platform devices we are 
forced to give them the benefit of the doubt. For DT systems the only 
vaguely-reasonable heuristic to distinguish between "platform" meaning 
"SoC memory-mapped device" and "platform" meaning "random crap made up 
by Linux" is whether the device has a populated OF node, but MFD's trick 
of hanging the parent device's OF node onto its synthesised children 
kicks a hole right through even that.

Modulo any other concerns with the existing code, does the change below 
make things work the way you want? It's still a bit of a bodge, but 
short of invasive large-scale changes with bus types I don't see a way 
to do the 'right' thing :/

Robin.

----->8-----
diff --git a/drivers/mfd/mfd-core.c b/drivers/mfd/mfd-core.c
index f5a73af60dd4..1e4a6e8bd630 100644
--- a/drivers/mfd/mfd-core.c
+++ b/drivers/mfd/mfd-core.c
@@ -138,7 +138,7 @@ static int mfd_add_device(struct device *parent, int id,

  	pdev->dev.parent = parent;
  	pdev->dev.type = &mfd_dev_type;
-	pdev->dev.dma_mask = parent->dma_mask;
+	pdev->dma_mask = parent->dma_mask ? *parent->dma_mask : 0;
  	pdev->dev.dma_parms = parent->dma_parms;
  	pdev->dev.coherent_dma_mask = parent->coherent_dma_mask;